Descending cortical inhibition affecting the respiratory control centers in the medulla, particularly under hypoxic conditions, has been observed. The present series of experiments seeks to identify the particular regions involved and to examine the neurotransmitter qualities in these regions followng long-term hypoxic exposure. Descending facilitation from the diencephalon largely overrides the cortical influence in hypoxia. Neurotransmitter concentrations and turnover in the diencephalon are being studied in long-term hypoxic exposure. The long-term goal is to understand the role of suprapontine influences on the chemical control of breathing.